The present invention, in some embodiments thereof, relates to energy conversion, and, more particularly, but not exclusively, to a photovoltaic cell comprising a doped semi-conductive substrate, and to methods of producing same.
Photovoltaic cells are capable of converting light directly into electricity. There is considerable hope that conversion of sunlight into electricity by photovoltaic cells will provide a significant source of renewable energy in the future, thereby enabling a reduction in the use of non-renewable sources of energy, such as fossil fuels. However, despite world-wide demand for environmentally friendly renewable energy sources, the high cost of manufacture of photovoltaic cells, as well as their limited efficiency of conversion of sunlight to electricity, has so far limited their use as a commercial source of electricity. There is therefore a strong demand for photovoltaic cells which are relatively inexpensive to produce, yet have a high efficiency.
Photovoltaic cells commonly comprise a p-type silicon substrate doped on one side thereof with an n-dopant (e.g., phosphorus) so as to form a n+ layer, and doped on the other side thereof with a p-dopant (e.g., boron) so as to form a p+ layer, thereby forming a n+-p-p+ structure. If an n-type silicon substrate is used, an n+-n-p+ structure is formed.
Electrical contacts are then applied to each side. Electrical contacts must cover only a small fraction of the surface area in order to avoid impeding the passage of light. Electrical contacts are typically applied in a grid pattern in order to avoid covering much of the surface area. Monofacial photovoltaic cells have such a grid pattern on one side of the photovoltaic cell, whereas bifacial photovoltaic cells have such a pattern on both sides of the photovoltaic cell, and can therefore collect light from any direction.
Efficiency may be improved by reducing reflectance of light from the surface of the photovoltaic cell. Methods for achieving this include texturing the surface and applying an antireflective coating.
In addition, attempts to improve efficiency include producing photovoltaic cells with a selective emitter, in which the n+ layer is more heavily doped in regions underlying electrical contacts, in order to decrease contact resistance.
German Patent No. 102007036921 is illustrative of such an approach, disclosing a method of producing a solar cell with an n+-p-p+ structure, in which a masking layer having openings corresponding to the pattern of the contact grid is used while doping with phosphorus, so that the concentration of phosphorus will be highest under the contact grid.
U.S. Pat. No. 6,277,667 discloses a method of manufacturing a solar cell using screen printing to apply an n-dopant source to form n+ regions, while a low dose n-dopant source is used to form shallowly doped n− regions. Electrodes are then screen-printed onto the n+ regions.
U.S. Pat. No. 5,871,591 discloses diffusing phosphorus into a surface of a silicon substrate, metallizing a patterned grid onto the phosphorus-doped surface, and plasma etching the phosphorus-doped surface, such that the substrate below the electrical contacts is masked and material that is not masked is selectively removed.
Another approach to achieving an n+ layer that is more heavily doped in regions underlying electrical contacts is the use of self-doping electrodes.
For example, U.S. Pat. No. 6,180,869 discloses a self-doping electrode to silicon formed primarily from a metal alloyed with a dopant. When the alloy is heated with a silicon substrate, dopant is incorporated into molten silicon.
Russian Patent No. 2139601 discloses a method of manufacturing a solar cell with an n+-p-p+ structure, by high-temperature processing of a silicon substrate with a borosilicate film applied to the back side thereof and a phosphosilicate film applied to the front side thereof. Removal of a layer of silicon from the front side of the substrate and texturing of the front side is then performed in one procedure. An n+ layer is then formed on the front side, followed by formation of contacts.
Additional background art includes U.S. Pat. Nos. 6,825,104, 6,552,414, European Patent No. 1738402 and U.S. Pat. No. 4,989,059.